In recent years, in mobile communication, various information such as image and data as well as speech are subjected to transmission. With this trend, demands for high-reliability, high-speed transmission are increased. However, when high-speed transmission is performed in mobile communication, the influence of delayed waves by multipath is not negligible, and transmission performance degrades due to frequency selective fading.
Multicarrier communication such as OFDM (Orthogonal Frequency Division Multiplexing) is focused as one of counter techniques of frequency selective fading. Multicarrier communication is a technique of performing high speed transmission by transmitting data using a plurality of subcarriers of transmission rates suppressed to such an extent that frequency selective fading does not occur. Particularly, the OFDM scheme utilizes a plurality of subcarriers orthogonal to each other where data is arranged, provides high frequency efficiency in multicarrier communication, can be implemented with relatively simple hardware, is particularly focused and is variously studied. Therefore, the OFDM scheme has attracted attention as a communication method to be employed in cellular scheme mobile communication, and has been studied variously. Further, according to the OFDM scheme, to prevent intersymbol interference (ISI), the tail ends of OFDM symbols are attached to the heads of OFDM symbols as a CP (Cyclic Prefix). By this means, on the receiving side, it is possible to prevent ISI as long as the delay time of delay waves stays within the time length of CP (hereinafter “CP length”).
On the other hand, recently, multicast communication has been studied. Multicast communication is not one-to-one communication such as unicast communication, but is one-to-many communication. That is, in multicast communication, one radio communication base station apparatus (hereinafter “base station”) transmits the same data to a plurality of radio communication mobile station apparatuses (hereinafter “mobile stations”) at the same time. By this multicast communication, in mobile communication systems, for example, distribution services of music data and video image data and broadcast services such as television broadcast are realized. Further, services using multicast communication are assumed to be services for relatively wide communication areas that cannot be covered by one base station, and, consequently, multicast communication entirely covers wide communication areas by transmitting the same data from a plurality of base stations. By this means, data for multicast communication (multicast data) is transmitted through a multicast channel shared by a plurality of base stations. That is, multicast data is the same between a plurality of cells. Thus, in the multicast channel, the same data is transmitted from a plurality of base stations at the same time, and, consequently, a mobile station nearby the cell boundary receives mixed multicast data comprised of multiple multicast data from a plurality of base stations.
Here, if the OFDM scheme is employed in multicast communication and there is a mobile station located nearby the cell boundary, if a plurality of the same OFDM symbols transmitted at the same time from a plurality of base stations with a shorter time lag than the CP length, these OFDM symbols are combined and received in a state their received power is amplified. To correct the channel fluctuation (phase fluctuation and amplitude fluctuation) of the combined signals by channel estimation, channel estimation values for these combined signals are necessary. Therefore, in multicast communication using the OFDM scheme, for the pilot used to calculate channel estimation values, the same pilot needs to be transmitted from a plurality of base stations at the same time, as in the case of multicast data. That is, the pilot for multicast data needs to be common between a plurality of cells.
On the other hand, in the unicast channel, a plurality of base stations transmit respective data (unicast data) (see Non-Patent Document 1). That is, unicast data differs between multiple cells. By this means, for the pilot used to calculate channel estimation values in unicast communication, different pilots need to be transmitted from a plurality of base stations as in the case of unicast data. That is, unicast data pilots need to be different between multiple cells.
Further, studies are underway to frequency-domain-multiplex multicast data and unicast data using the OFDM scheme (see Non-Patent Document 2). The description in this document employs broadcast instead of multicast. While multicast communication employs a mode of communication in which information is transmitted only to specific mobile stations subscribing services such as news groups, broadcast communication employs a mode of communication in which information is transmitted to all mobile stations as in today's television broadcast and radio broadcast. Further, a combination of multicast and broadcast is sometimes referred to as “MBMS (Multimedia Broadcast/Multicast Services channel.”    Non-Patent Document 1: 3GPP RAN WG1 LTE Adhoc meeting (2005.06) R1-050589 “Pilot channel and scrambling code in evolved UTRA downlink”    Non-Patent Document 2: 3GPP RAN WG1 Meeting #43 Seoul, Korea, 7-11 Nov., 2005, R1-051342, “Multiplexing of Broadcast and Unicast Traffic”